This invention relates to melt-blowing processes for producing non-woven mats. More particularly, it relates to processes in which a fiber-forming thermoplastic polymer resin is extruded in molten form through orifices of a heated nozzle into a stream of hot gas to attenuate the molten resin as fibers which form a fiber stream, the fibers being collected on a receiver in the path of the fiber stream to form the non-woven mat.
Various melt-blowing processes of the foregoing description have been described heretofore, earlier efforts including those of Hall (U.S. Pat. Nos. 2,374,540), Manning (2,411,659; 2,411,660; and 2,437,263) and Marshall (2,508,462). A melt-blowing process is disclosed in the article "Super-Fine Thermoplastics," by Van A. Wente, in Industrial and Engineering Chemistry, Volume 48, No. 8, (1956), pages 1342-1346 and also in Naval Research Laboratory Report No. 111437, submitted Apr. 15, 1954, entitled "Manufacture of Super-Fine Organic Fibers." The Naval Research Laboratory process is further described in NRL Report 5265, dated Feb. 11, 1959, and entitled "An Improved Device for the Formation of Super-Fine, Thermoplastic Fibers." U.S. Pat. No. 3,532,800 to Wyly et al discloses a use of the Naval Research Laboratory melt-blowing process. A melt spinning and blowing process is disclosed in British Pat. No. 1,055,187 and U.S. Pat. Nos. 3,379,811 and 3,502,763. As evidenced by these prior melt-blowing processes, it has been believed and taught that degradation of a fiber-forming thermoplastic polymer resin is to be avoided in a melt-blowing process.
Heretofore, non-woven mats made of essentially discontinuous fibers and produced by known melt-blowing processes have contained undesirable coarse "shot" or "beads" of material larger than about 0.3 millimeter in diameter. Moreover, prior melt-blowing processes operate at low and generally uneconomical resin flow rates of less than 1.0 gram per minute per resin outlet and experience difficulty in producing soft, fine, high quality mats that do not contain coarse shot. Also, earlier melt-blowing processes do not disclose how to produce mats substantially free of coarse shot from a fiber-forming thermoplastic polymer resin having a high intrinsic viscosity (1.4 or greater), particularly with C.sub.3 -C.sub.8 polyolefins, especially polypropylene. These polyolefins, which are conventionally produced in the presence of a heterogeneous solid catalyst, normally have very high intrinsic viscosities typically 2.2 to 4 higher, corresponding to high viscosity average molecular weights of about 270,000 to about 550,000 and higher. Intrinsic viscosities as used herein are measured in decalin at 135.degree. C. The melt flow rates or melt indexes of these high intrinsic viscosity resins are quite low, typically about 5 to 0.5 and lower.